Nowadays, a standardized cell from a cell library is used in circuit design. The standardized cell has either a cell height equal to the height of other cells in a same cell row, or has a cell height that is a multiple of a standard cell height. A cell having a single cell height is normally engaged to only one power/ground (P/G) rail set. However, especially for electro migration (EM) and voltage (IR) drop analysis, it becomes more complicated for. P/G rail planning when a cell having a cell height that is a multiple of the standard cell height.